1. Field of the Invention
The present invention relates to a developing device and an image forming apparatus, such as an electrophotographic apparatus or electrostatic printer, which is provided with a developing device for visualizing an electrostatic latent image and a toner cartridge for replenishing the developing device with toner.
2. Description of the Related Art
Recently, there have been provided many image forming apparatuses which incorporate a removable process unit. In general, process units integrally comprise an image carrier, a developing device, a cleaner for clearing the image carrier of the toner remaining thereon after transfer, and a waste toner receiving section for receiving the toner removed by means of the cleaner. Among these process units, those which use a removable toner cartridge for replenishing the developing device with the toner have an advantage over those which are furnished with an integral toner cartridge in entailing lower initial cost.
Generally, moreover, life management of the process unit is based on its lifetime print quantity or frequency of toner-empty detection. If the lifetime print quantities for the process unit and each toner cartridge are 10K and 5K, respectively, the life is determined depending on the detection of the attainment of 10K by the print quantity or a second detection of a toner-empty state, whichever is earlier.
Usually, in this case, the lifetime print quantity is a value for standard printing (with print ratio of 5%, for example), so that it is inevitably subject to variation depending on the user of the apparatus. The frequency of toner-empty detection is a question here. If the user replaces the toner cartridge before the toner-empty state is established, for example, then the toner-empty state cannot be detected, so that the life of the process unit will be determined solely by the print quantity.
No problem will be aroused, in this situation, if the user employs an average print ratio lower than the standard print ratio. If the user's average print ratio is higher than the standard ratio, however, the printing entails a large toner consumption, unlimited in an extreme case.
Thus, in the process unit which contains therein the receiving section for receiving the waste toner removed by the cleaner for clearing the image carrier of the toner remaining thereon after transfer, the capacity of the waste toner receiving section must be made so large that the process unit itself is very bulky.
To cope with this, a method for detecting the frequency of toner cartridge mounting is conventionally proposed. According to this method, however, the apparatus body should be provided with a detecting section for the cartridge mounting frequency besides the toner-empty detecting means. With use of this frequency detecting section, however, the life management cannot be effected if a unicolor process unit is used in the middle of the operation. In this case, therefore, the developing device must be also furnished with a detecting mechanism for the toner cartridge mounting frequency. Inevitably, therefore, the process unit is complicated and expensive.
In order to miniaturize the process unit, the size of the toner storage section must be minimized. If the capacity of this storage section is made smaller than that of the toner cartridge, however, the toner may spill from the process unit and soil the apparatus body or its surroundings when the cartridge is replaced before the toner-empty state is detected.
As mentioned before, furthermore, the life management of the conventional process unit is based on its lifetime print quantity or frequency of toner-empty detection. In the case where the lifetime print quantity is used as the criterion, in particular, the user of the apparatus who employs a low print ratio may possibly replace the process unit whose printing capacity is not exhausted yet, thus suffering a low economical efficiency.